Molecular tailoring of surfaces

ABSTRACT

This invention describes a new approach to three-dimensional molecular tailoring of surfaces. In this process, a plasma deposition step is initially employed to deposit reactive functional groups on the surface of a solid substrate. This is then followed by immersion of the coated substrate in a solution during which time solute molecules react with the functional surface groups introduced during the plasma process. Solute molecules are attached to the surface during this second step. This simple two-step process is of general utility in that both the nature of the plasma introduced surface group and the nature of the solute molecules can be varied. Additionally it is possible to provide exact control of the surface density of reactive groups introduced during the plasma process and thus the concentration of solute molecules coupled to the solid surfaces. A particularly significant aspect of this invention is that the second step chemical derivatization reactions can be carried out using aqueous solutions at room temperature. The RF plasma polymerization of substituted perfluorohexenes is shown to produce films having unusually high —CF 3  content. These films are produced under both pulsed and continuous-wave plasma deposition conditions. The relative —CF 3  content of these polymers increases with decreasing average RF power absorbed during the film formation processes. The films produced under the least energetic condition (i.e., pulsed plasma, 0.1 ms on/3.0 ms off and 100 watts peak power) are exceptionally hydrophobic, exhibiting advancing and receding water contact angles in excess of those observed with Teflon® surfaces. The most hydrophobic films have a —CF 3  content which represents 40% of the carbon atoms present in the sample.

This is a continuation of application Ser. No. 08/632,935 filed Apr. 16,1996, now U.S. Pat. No. 5,876,753.

The U.S. Government has certain rights in the present invention pursuantto the National Institutes of Health under Grant #R01AR43186-01 and bythe Texas Higher Education Coordinating Board ATP Program under Grant#003656-105.

FIELD OF THE INVENTION

This invention provides a new approach to molecular tailoring ofsurfaces.

BACKGROUND OF THE INVENTION

The chemical composition of surfaces plays a pivotal role in dictatingthe overall efficacy of many devices. Applications in which surfacechemistry exerts a major influence in device performance include thebiocompatibility of materials, biosensors, heterogeneous catalysis, andthe permselectivity of membranes, to mention but a few of the many suchexamples. In recognition of the important role exerted by surfaces,surface modification research has represented an exceedingly active areafor many years. A wide range of techniques, involving both physicallyand chemically oriented approaches, has been developed in efforts toprovide specific improvements of surfaces. Examples of such work includevarious vapor deposition methods, plasma processes, sputteringtechniques, chemical etching processes, ion implantation, etc. In theseprocesses, the surface treatments are designed to improve deviceperformance and/or reduce costs involved in preparation of substratesurfaces. An enormous patent literature exists describing the varioussurface modification techniques which have been developed and the manyapplications which have been identified.

Despite the extensive work in this area, a significant need remains forimproved methods to control surface modification processes at themolecular level. Unfortunately, the majority of currently employedsurface modification techniques provide unsatisfactory controllabilityof the film chemistry during the deposition process. Additionally,current surface modification techniques, as employed on industrial scaleoperations, are restricted to planar, 2-dimensional surface layers.

In recognition of the need for improved molecular level film chemistrycontrol, and particularly the requirement for three-dimensionallymolecularly designed surfaces, additional approaches to surfacemodification have been described in recent literature. For example, theuse of self-assembling monolayers (SAM's) have been extensivelydeveloped during recent years to provide layered structures havingthree-dimensional molecular properties. A second developing approach tosurface modifications, and one of direct relevance with respect to thepresent invention, is use of an initial surface treatment to introducereactive functional groups which are then subjected to subsequentchemical derivatization processes. These multi-step procedures,ultimately resulting in coupling of specific molecules to the surface,can provide what may be described as three-dimensional molecularlytailored surface.

Although these new surface modification procedures have produced someinteresting results, the overall processes described to date areexceedingly time-consuming, inefficient and require the use ofundesirable hazardous solvents. Descriptions of recent literaturereports of these surface modification processes clearly reveal thecomplex procedures involved in attempting to couple specific moleculesto surfaces. For example, recent work has employed a plasma depositionmethod for introduction of surface hydroxyl (—OH) groups, which are thenreacted in a second step to provide covalently bonded molecules tetheredto the surface (Ranieri et al.). In this process, the plasma-introducedsurface —OH groups are first reacted with di-imidazole, with thisreaction being carried out in dry tetrahydrofuran at room temperaturefor 30 minutes. This was then followed by an aqueous solution reactionat 4° C. for 72 hours at a pH of 8.4 to covalently attach low molecularweight peptides to the surface. This process requires both the use of annonaqueous solvent and an extraordinarily long (i.e., 72-hour) couplingreaction to couple the desired molecules to the surface (Ranieri etal.).

Alternately, a chemical process can be employed to introduce the surfacehydroxyl groups which are then subsequently derivatized. For example, acomplex procedure has been employed to provide a so-called glycol phasesurface which contains reactive —OH groups (Massia et al.). In thisprocedure, glass coverslips are initially soaked in 0.5 M NaOH for 2hours, then rinsed in deionized water and then immersed in pH 5.5aqueous solution of (3-glycidoxypropyl)trimethoxy-silane. This reactionwas maintained for 2 hours after which time the pH was adjusted to 3.0followed by heating again for 1 hour to convert the oxirane moieties onthe derivatized glass to glycol groups. This was followed by reaction ofthe surface hydroxyls with tresyl chloride using an acetone solventfollowed by rinsing with 1 mM HCI and immersion in 0.2 M NaHCO₃ bufferat pH 9. Coupling of the desired peptides to the glass surface was thencarried out via a 20-hour incubation reaction to the sulfonyl-containingsurfaces.

Still other workers have employed a somewhat different but equallycomplex route to obtain the glycophase glass surface (Clemence et al.).In this work, they employ a sequence in which the glass coverslips areincubated for 5 minutes in a boiling solution of NH₃/H₂O₂/H₂O. Afterrinsing with distilled water, the glass disks were rinsed in acetoneafter which they were reacted with a solution containing CPTMS and TEAin dry toluene. This was followed by successive washes in chloroform,acetone and methanol and then dried in vacuum. This was followed by aHCl rinse, then incubation for 60 min. at 90° C. in 1 mM HCl followed byrinsing with doubly distilled water to obtain the glycophase glass.These workers then employed a photochemical method to attach peptidescovalently to the hydroxylated surfaces. This technique requires theinitial coupling of a photochemical label to the peptides. Thephotochemical labels employed wereN-{m-[3-(trifluoromethyl)-diazirin-3-yl]phenyl}-4-maleimidobutyramide or4-maleimidobenzophenone. With either label a complicated reactionprocedure is required to attach them to the peptide with the synthesisrequiring the use of HPLC to separate the desired complex from thereaction mixtures. These photolabeled molecules are then subsequentlyattached to hydroxylated surfaces using a photochemical technique.

Alternately, biomolecule surface immobilization has been reported usingbifunctional photochemical sensitive molecules to achieve this goal(Sigrist et al.). In this approach, a heterobifunctional crosslinker isemployed to link the biomolecules to a surface. For example,3-(trifluoromethyl)-3-(m-isothiocyanophenyl) diazirine is initiallycoupled to a surface containing amine groups via coupling through theisothiocyano group. Subsequent photolysis of the surface attacheddiazirine generates a carbene radical which can react with biomolecules,if the biomolecules are within molecular vicinity at the time of carbenegeneration. As noted by these authors: “If target molecules are notpresent during the carbene lifetime, the intermediate will react withevery molecular species present including water.” This of course leadsto relatively low surface immobilization of the protein molecules as theprocess selectivity is relatively low. Thus this recent biomoleculesurface immobilization process requires synthesis of a complexheterobifunctional photochemical sensitive intermediate linker molecule,attachment of this linker to a functionalized surface and, finally,photochemical attachment of this linker to the biomolecules. Over allthe process is complex and it results in relatively low yields and lowselectivity of biomolecule attachment to surfaces.

The above results have been cited to document the relatively complexreaction procedures currently being employed to covalently attachmolecules to surfaces. The research cited above is from leadingresearchers and includes literature citations as recent as 1995. Thus itseems accurate to conclude that the complex techniques employedrepresent “state-of-the-art” in surface modifications in which moleculesare attached to activated surfaces.

The process of the present invention is to be contrasted with numerousearlier applications of plasma surface modifications to enhance theinteraction of solid substrates with other molecules and materials.Earlier plasma depositions were employed to improve the adsorptionand/or adhesion of various molecules to the modified surfaces. Forexample, U.S. Pat. Nos.5,055,316 (Hoffman et al.) and 5,258,127 (Gsellet al.) both employed plasma surface modification to enhance adsorptionof various biomolecules. In a similar vein, U.S. Pat. No. 5,178,962(Miyamoto et al.) utilized a plasma discharge process to change thechemistry of a macromolecular synthetic resin film by exposure toexcited plasma species to generate surface active groups, which are thencoupled to metal atoms to form a metallic outer layer. This latter workinvolved non-polymerizable gases and the metal films were deposited onthe plasma activated surfaces by high energy vapor deposition processes.

One way that the present invention differs from the above notedtechniques in that the initial plasma surface modification step isdirectly followed by a chemical derivatization process in which desiredmolecules are covalently bound to the surfaces via simple chemicalreaction.

SUMMARY OF THE INVENTION

The present invention focuses upon a process for preparing a solidsurface attached to a target material. The process includes fixing acarbonaceous compound having a reactive functional group to a surface bylow power plasma deposition. As used herein, the term “carbonaceouscompound” means an organic compound comprising carbon. Such low powerplasma deposition permits retention of functional group activity. Atarget material may then be added directly to the activated surface byreaction with the reactive functional group, preferably in a singlestep. This results in a target material covalently bonded to thecarbonaceous compound affixed by plasma deposition to the solid surface.The reacting step for coupling the target molecule is preferably areaction carried out with a solution of target material. The solvent forthe solution of target material is preferably water. Affixing of thecarbonaceous compound having a reactive functional group is carried outby low power plasma deposition, most preferably a variable duty cyclepulsed plasma deposition with higher powers being utilized for initialdepositions on the surface. Such variable duty cycles permit thedeposition of carbonaceous compounds with reactive functional groupsthat control concentration. The reactive functional groups of thecarbonaceous compounds include carbon-halogen, acid halide, acidanhydride, sulfhydryl, phosphide, carboxylic acid, aldehyde and ketone,for example, although others may be utilized as well. The halide ispreferably chloride and the halogen is preferably bromine or iodinealthough chlorine may be used.

The substrates utilized may be any solid surface including polymer,ceramic, metal, silanized metal, carbon, fabric, glass, silanized glass,semiconductor, wood, rubber, paper, hydrogel, cellulose or composite.The solid substrates also include films, particularly polymeric filmssuch as polysilicones, polyolefins, and many others too numerous tolist.

Another important aspect of the present invention is the plasmadeposition of a hydrophobic and substantially —CF₃ dominated perfluorocompound film. The preferred perfluoro compound is a perfluorocarbonsuch as the most preferred perfluorinated trifluoromethyl substitutedperfluorohexene. To form a perfluorinated surface also having a reactivesurface, a perfluorinated compound is mixed with a carbonaceous compoundhaving a reactive functional group such as an akenyl or alkyl halide,isothiocyanate, cyanide, benzene, acetate, mercaptan, glycidyl ether,ether, chloroformate, methyl sulfide, phenyl sulfone, phosphonicdichloride, trimethylsilane, triethoxysilane, acid, acid halide, amine,alcohol, or phosphide. The target materials may include any substancecapable of reacting with the reactive functional groups. Preferredtarget materials include amino acids, fluorinated amino acids, proteins,peptides, saccharides, hormones, hormone receptors, polynucleotides,oligonucleotides, carbohydrates, glycosaminoglycans (such as heparin,for example) polyethylene glycol and polyethylene oxide. Derivatives ofall these various target materials may be prepared and still retainreactivity with one or more of the active functional groups such thatthey may be attached to an activated surface. In one aspect the presentinvention involves producing a surface with reduced adherence forbiological materials. Surfaces with coupled polyethylene glycol,polyethylene oxide or abundant —CF₃ groups are among the most preferredsubstituents for producing a surface with reduced adherence forbiological material. Since biological materials may be any of thecomponents of circulating blood, particularly its protein and cellularcomponents, although conceivably antibacterial or antimarine organismmaterials could be added as well. The present invention concerns thefirst production of a solid surface having a pendant bromoalkenyl, acidchloride or carboxylic acid group suitable for coupling to a targetmaterial. For purposes of perfluorinated surfaces, the present inventionincludes a solid surface having pendant perfluorinated groups where thecarbon in CF₃ represents more than about 40% of the carbon present onthe surface. A surface has been produced having a pendant perfluorinatedcarbonaceous compound with a water contact angle greater than about120°.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1D shows high resolution C(1s) X-ray photoelectron spectra(XPS) of plasma polymerized films from allyl bromide monomer duringcontinuous wave (CW) and pulsed plasma deposition.

FIGS. 2A-2C. High resolution C(1s) XPS spectra of pulsed plasmapolymerized films of the C₉F₁₈ monomers. RF duty cycles and peak poweremployed are shown for each film.

FIGS. 3A-3B. High resolution C(1s) XPS of polymerized C₉F₁₈ filmsobtained under CW plasma conditions of 50 and 5 watts, as shown.

FIG. 4 Comparison of advancing and receding water contact angles for lowpower pulsed plasma and CW pl asma C₉F₁₈ synthesized films relative to aTeflon® film standard.

FIG. 5. Illustration showing the non-wettability of a —CF₃ dominantsurface as produced via low power plasma polymerization of C₉F₁₈monomer.

FIG. 6 shows a high resolution ESCA spectrum of an allyl bromide surfacecoupled with hexafluoro-DL-valine.

FIG. 7A and FIG. 7B which show a high resolution ESCA spectrum of anallyl bromide surface coupled with YIGSR.

FIGS. 8A-8D show a high resolution C(1s) spectra of silicon surfacesubstrates with acryloyl chloride deposits at various radiofrequencyduty cycles.

FIG. 9 shows relative amounts of acid chloride functionality retained inthe surface film as a function of average radiofrequency power.

FIG. 10 shows an ESCA curve fit analysis of the C(1s) high resolutionspectrum of a film obtained from plasma deposition of acryloyl chlorideshowing the complete assignment of both chlorine and non-chlorinecontaining functional groups including the highest binding energy—COClpeak.

FIGS. 11A-11D show high resolution C(1s) ESCA spectra from acrylic acidfilms formed at varying plasma deposition duty cycles.

FIG. 12 shows FT-IR absorption spectra of a plasma-deposited acrylicacid films formed at several plasma duty cycles.

FIG. 13 shows the increased incorporation of surface nitrogen from thecovalent reaction of cysteine molecules with an allyl bromide film on aPET substrate as a function of the reaction time between the cysteinesolution and the coated PET substrate.

FIG. 14 shows the C(1s) high resolution ESCA spectrum after reaction ofa PEO-NH₂ polymer (MW{tilde over (=)}5000) from aqueous solution with anallyl bromide film previously plasma deposited on a PET substrate.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention provides a significantly simpler and moreversatile approach to surface modification science to providethree-dimensional molecular tailoring of solid substrates. Thisinvention involves a simple, two-step procedure which allows forexcellent controllability of the surface density of the moleculestethered to a surface. This new procedure significantly reduces the timerequirement for surface tailoring and also eliminates the use of anyhazardous solvents. Thus, it provides the first approach to moleculartailoring of surfaces which is useful in terms of large-scale,industrial type, applications.

Synthesis of a novel new fluorocarbon film is enabled. This film isobtained by plasma polymerization of a trimer mixture of substitutedperfluorohexenes (C₉F₁₈) or the like. Use of low duty cycle pulsed orlow energy continuous-wave plasmas provides films in which —CF₃ groupsare the dominant functional group present. These —CF₃ rich films areexceptionally hydrophobic as revealed by water contract anglemeasurements.

As in some of the previously noted work, the first step in the presentnew surface modification process involves an initial plasma treatment.However, an important distinction between the present invention andtechniques of previous workers, is that the present invention involvesdeposition of surface functional groups having significantly morechemical reactivity than those employed by previous workers. Forexample, the prior art noted earlier (Ranieri et al.; Massia et al;Clemence et al.) involved introduction of surface —OH groups whichrequired complex and energetic reactions for attachment of intermediatemolecules for subsequent attachment to biomolecules. On the other handother groups (Sigrist et al.) employ —NH₂ functionalized surfaces which,as noted earlier, also requires complex procedures to achievebiomolecule surface attachment. Neither the —OH or —NH₂ surface groupspossess the degree of reactivity towards other reactants, particularlynucleophiles, for example, as the surface active groups (e.g., C—Br,C—COCl, C—I, C—SH, C—COOH, etc.) of the present invention. Additionally,the present invention is able to provide unusual film chemistrycontrollability during the initial plasma deposition step which permitscontrol of both the nature and surface density of the functional groupsobtained from a given monomer. By introducing chemically more reactivefunctional groups than heretofore reported, molecules may be covalentlycoupled to these functionalized surfaces using unusually mild reactionconditions and short reaction times.

Important principles involved in the present invention can beillustrated with reference to a specific surface modification. In thisprocess, a low pressure pulsed plasma polymerization is initiallycarried out using allyl bromide as the reactant monomer to deposit thinfilms on targeted substrates. By varying the duty cycle it is possibleto control the bromine to carbon atom ratio in these films to a veryexact degree. High resolution C(1s) electron spectra (XPS) were used toanalyze plasma polymerized films obtained from allyl bromide monomerduring continuous wave (CW) and pulsed plasma deposition runs using 200watts power. The pulsed plasma depositions were carried out at, e.g.,on-off duty cycles of ⅗, {fraction (3/15)}, {fraction (3/45)} and 3/60ms. There was a sharp increase in the bromine content of the film as theduty cycle during deposition is decreased. For example, an approximate400% increase in the film content of bromine atoms, relative to thecarbon atoms, is observed in comparing the pulsed run at the {fraction(3/60)} ms duty cycle to the CW result.

The carbon-bromine bonds and other active functional groups introducedby the plasma surface treatment have a reactivity which permits readycovalent attachment of molecules to these surfaces via facile one-stepprocesses. Furthermore these derivatization processes may be carried outin aqueous solution, at room temperature and with relatively shortreaction times. A variety of nucleophilic displacement reactions havebeen carried out in which various molecules have been covalently bondedto the surface via displacement of, for example, the bromine atoms. Anexample of this reaction is shown for surface attachment of an aminoacid via the reaction:

Surface-C—Br+NH₂CH₂CH₂COOH→Surface-C—NH—CH₂CH₂COOH+HBr  (1)

Similar type reactions can be carried out for a wide range ofnucleophiles or coupling agents. Thus, as illustrated above, it ispossible to avoid the complex chemistry of current molecular tailoringpractices via introduction of more active surface functional groups.

The C—Br groups mentioned in this illustration represent but one of arange of reactive surface groups which can be employed in this newmolecular tailoring procedure. Other examples of surface moieties whichalso provide the reactivity required for this purpose include optionalleaving or coupling groups such as, for example, other halogens (Cl andI), carboxylic (—COOH), acid halide (—COX, where X represents ahalogen), anhydrides [C(O)OC(O)] groups, thiol (SH) groups, aldehydes(—CHO), ketones (CH₂═O), and the like. Using either the low duty cyclepulsed or low power CW plasma deposition technique and appropriatemonomer, the present invention has succeeded in deposition of thesegroups on solid surfaces. These groups were then readily reacted withvarious nucleophiles, again using a simple one-step coupling processcarried out at ambient temperatures.

An important component of the present invention involves the successfulretention of the reactive groups of the monomers during the plasmainduced film formation processes. In general these functional groups(e.g. —COCl, —COOH, —CHO) represent chemical structures which are alltoo readily destroyed during energetic plasma polymerization processes.In many cases, this destruction reflects the favorable energeticsinvolved in formation of products such as CO or CO₂ which thus representloss of the key functionality of the monomers. However, as documented inthe present invention, it is clearly possible to retain these reactivegroups in the plasma modified films if unusually low powers are employedduring the plasma deposition steps. However, as anyone who is wellversed in the practice of plasma polymerizations will recognize, thecorrelation between applied electromagnetic power and the plasmagenerated film composition is complicated by the fact that many otherprocess variables must also be simultaneously considered. Theseadditional variables include such factors as the size (e.g., volume) ofthe reactor chamber, the location of the substrates relative to theplasma discharge zone, the monomer flow rates, the monomer pressure, thenature of the monomer, etc. For example, it is well known that increasedmonomer functional group retention can be maintained at a given power byincreasing the monomer flow rate (Yasuda). Likewise, the use of a largereaction volume at a given applied power would also provide increasedretention of monomer functional groups, as this variation in effectdecreases the power density during the plasma polymerization process.Similarly there are many studies which have shown that location ofsubstrates downstream of the electrodes employed to power the dischargewill provide surface coatings having increased incorporation of monomerfunctional groups. This observation reflects the fact that as thesubstrate is moved progressively away from the discharge zone there is apronounced decrease in the intensity of the electromagnetic field andthus development of less harsh reaction conditions. In other words,although a relatively high power may be employed to power the dischargebetween the electrodes, the effective power governing the chemistryeither up-stream or down-stream of these electrodes would, in fact, beextremely low. Nevertheless significant film formation has been observedat these plasma remote positions.

In particular, quantitation of the power requirement is severelycomplicated by the strong dependence of functional group retention onthe nature of the monomer (and functional group) and the power employed.This dependency is clearly illustrated in the present invention incomparing the power dependence of Br atom retention in the allyl bromideplasma polymerization to that observed in the retention of the —COClgroup during polymerization of acryloyl chloride. With respect to thiscomparison, we introduce the concept of average power, which for pulsedplasma depositions is defined as:${\langle{{Avg}.\quad {Power}}\rangle} = {\frac{{plasma}\quad {on}\quad {time}}{{plasma}\quad ( {{on} + {off}} )\quad {time}} \times {peak}\quad {RF}\quad {power}}$

As shown in FIG. 1, it is possible to retain large portions of the C—Brfunctionality of the allyl bromide monomer in a pulsed plasma run of 3ms on and 15 ms off at a peak power of 200 watts. (FIG. 1C). Thiscorresponds to an average power, as defined above, of 33 watts. Underthis condition, the plasma generated film contains one bromine atom forevery 3.94 carbon atoms. This can be contrasted with one bromine foreach 3 carbons in the starting monomer, thus representing a significantretention of Br atoms in the plasma deposited film. In contrast, theexperiments with acryloyl chloride, in the same reactor and atessentially the same monomer flow rate and pressure as employed withallyl bromide, results in essentially zero —COCl functional groupretention if plasma polymerization were carried out at an average- powerof 33 watts. (FIG. 9). In fact, it is only at average powers of lessthan approximately 5 watts that significant —COCl retention is observedduring plasma polymerization of this monomer under the particularcombination of experimental conditions employed, as shown in FIG. 9.Wide ranging variations in the minimum power requirements needed toretain functional group incorporation in the plasma generated films havebeen observed in comparing other monomers under similar depositionconditions. For example, it requires significantly lower average powerto retain C—I bonds in the plasma polymerization of allyl iodide versusthat observed with allyl bromide.

In light of the above considerations, it is simply not possible todefine a specific wattage value to be defined as “low wattage” for thepurpose of this invention. The fact of the matter is that the specificlow wattage value required to retain a specified percentage of monomerfunctional groups will vary with each monomer and with variations of theother processing variables, as noted above. Therefore, as those schooledin the art will recognize, the important feature is to adjust the plasmadeposition conditions to the point at which the desired incorporation ofreactive functional groups in the plasma deposited film is achieved.This desired incorporation can be achieved in many different ways, allof which have, for the sake of simplicity, been categorized under the“low power” concept. Thus, changing process variables such as thereactor volume, location of substrates, flow rate of the monomers [i.e.the W/F parameter (Yasuda)] are all understood to be able to providelower power deposition conditions as used in the context of the presentinvention.

In light of the above discussion, coupled with the documentation ofretention of the highly thermodynamically unstable —COCl and —COOHgroups in this invention, it is clear that the low power plasmapolymerization process, particularly the pulsed plasmas, can be utilizedto provide surface films having an enormous range of reactive functionalgroups. For example, in addition to the specific monomers identifiedabove and in the Examples, it is clear to those skilled in the art thatthe initial surface functionalization plasma step could be employed withvirtually any volatile monomer having a desired functional group. Thepresent invention clearly illustrates this fact with several monomershaving reactive groups which are normally loss under typical plasmapolymerization conditions (e.g. —COCl and —COOH). Nevertheless, it isshown herein that these reactive groups are retained as intact entitiesunder the exceptionally mild plasma deposition conditions employed inthis invention. Obviously this technology can be extended to include arich and diverse range of additional functional groups. For example, thefollowing list of allyl type compounds represents a partial listing ofadditional functional groups which could be deposited as intact entitiesfor further chemical derivatizations:

MONOMER FUNCTIONAL GROUP allyl isothiocyanate —NCS allyl cyanide —CNallyl benzene —C₆H₅ allyl acetate —COOR allyl mercaptan —SH allylglycidyl ether

allyl ether C—O—C allyl chloroformate —COOCl allyl methyl sulfide C—S—Callyl phenyl sulfone —SO₂C allylphosphonic dichloride —CP(O)Cl₂allyltrimethylsilane —Si(R)₃ allyltriethoxysilane —Si(OR)₃

The above partial listing has focused on allyl type compounds sincethese materials have favorable vapor pressures and our experience withother allyl compounds, as described in this invention, documents thefact that retention of these functional groups is clearly anticipatedunder low power plasma polymerization. Obviously this technology canalso be extended to non-allyl precursors, including even saturatedcompounds as shown in one of the Examples.

With respect to the present invention, the second step coupling reactionof molecules to the plasma treated surfaces, when conducted in asolution, imposes an obvious requirement on the nature of theplasma-introduced reactive functional groups. This requirement centerson recognition that solvolsis of these plasma introduced functionalgroups must be significantly slower than the competing reaction bysolution nucleophiles with these same groups if this invention is to beuseful in molecular surface tailoring processes. For example, in thecase of aqueous solution reactions, hydrolysis of these plasma depositedfunctional groups must be significantly slower than the competingnucleophilic displacement reactions. The present results clearly showthat this requirement is satisfied for selective surface functionalgroups, such as the C—Br group illustrated previously.

There are many applications for the technology described in thisinvention. Certainly one important applications area will be in thebiomaterials field, specifically providing surface modifications toimprove biocompatibility of these materials. In some of theseapplications surface modifications are required to improve the adherenceof biomolecules (e.g., proteins, glycosaminoglycan, cellular materials,etc.) to surfaces, particularly those surfaces involved with bloodcontact. The present proposal offers an unusually facile approach tostructuring surfaces which will promote these biomolecular adsorptionprocesses. Recent work has shown that attachment of specific peptidesequences (e.g., YIGSR and RGD) to surfaces is particularly effective inpromoting adsorption and growth of specific cells. The process describedin the present invention provides a simple approach to surfaceattachment of these peptides and any other peptide, where thisattachment is accomplished from aqueous solution. The aqueous solutioncapability is particularly significant in that this solvent permitsretention of the geometrical structures of these peptide molecules.These geometric considerations can be very important with respect topromoting biomolecule adsorptions. Alternately, it is clearly possibleto covalently attach biomolecules (such as saccharides, proteins, etc.)directly to the plasma functionalized surfaces via simple covalent bondformation. For example, the amino and sulfhydryl groups of moleculessuch as enzymes will couple directly to surface containing reactivegroups such as C—Br, C—COCl, etc. Similarly, biologically importantmolecules such as heparin will also bind to these surfaces via theiramino groups.

Clearly, the technology described in this invention can be employed withmolecules other than peptides, glycosaminoglycans and the like topromote improved biocompatibility of materials. For example, manybiomaterial applications require biologically non-fouling surfaces(i.e., surfaces which minimize or eliminate cellular or biomoleculeadsorptions). Research from many laboratories has shown thatsurface-attached polyethylene glycol (PEG) molecules are particularlyeffective in reducing biomolecule adsorptions. The surface modificationprocedure described herein is ideal for this purpose in that PEGmolecules can conveniently be bonded to the plasma modified surfaces.This may be an aqueous bonding process, taking advantage of thewater-solubility of PEG molecules. In a preferred embodiment, the PEGmolecules employed are terminated with —NH₂ groups which canconveniently be coupled to the plasma deposited functionalcarbon-bromine groups as shown herein. With other attached functionalgroups, e.g. acyl halide, PEG can be directly attached.

The biomaterials example represent but one of many applications for thisnew surface modification procedure. As those schooled in the art willrecognize, this process can be applied advantageously to any device inwhich surface chemistry exerts an important role in the device function.For example, this process can be employed to molecularly tailor surfacesfor improving the performance of devices such as sensors, heterogeneouscatalysts, permselective membranes, etc. It could also be employed forimproved coating applications such as those involved in anti-corrosionand passivation coatings, optical and dielectric coatings, etc. Eachsuch application simply involves the surface attachment of selectedmolecule(s) required to improve the performance of a particular device,using the simple two-step approach described herein.

An additional feature of the present invention is the moleculartailoring of surfaces where selective attachment of more than one kindof molecule is achieved. In this process, the initial plasma depositionis employed to simultaneously deposit two or more functional groups viause of appropriate monomer mixtures. Subsequently, selective chemicalattachment reactions are employed to attach different molecules to eachfunctional group introduced by the plasma deposition process.

The surface modification procedures described herein can be illustratedby reference to specific functional groups as involved in selectivesurface attachment of two different kinds of molecules. For example, theplasma process is employed to simultaneously introduce C—Br and —COClsurface groups via use of an appropriate monomer mixture such as allylbromide and acryloyl chloride. The —COCl acid chloride groups aresignificantly more reactive than the C—Br groups. For example, undermost conditions the acid chlorides, but not the carbon-bromides, reactwith alcohol molecules:

Thus selective attachment of the alcohol molecule can be carried out atthe acid chloride sites. Subsequently, solution molecules can beattached to the C—Br sites, using stronger nucleophilic reagents such as—NH₂ groups, as previously described in reaction (1). In this way, themolecular tailoring process can be conveniently employed to covalentlybind two different types of molecules to a particular surface. Further,the relative concentrations of the covalently bonded molecules to thesurface is easily controlled by simply varying the relative partialpressures of the two monomers employed during the plasma depositionprocess. It should also be noted explicitly that the R groups denoted inthe above coupling reactions may also contain specific functional groupswhich could be utilized in subsequent further coupling reactions to thesolid surfaces. As one of many such examples, the R group may contain anunsaturated bond which could be utilized for additional moleculartailoring reactions. Such a process is illustrated in reaction 4.

Clearly, the alkene appendage now affixed to the surface provides anadditional route to further important coupling reactions includingparticularly synthesis of catalytic materials. As those schooled in thisart will readily recognize, the nature of the R groups introduced inreactions such as (2) and (4) (i.e., during the initial couplingprocess) will provide a rich and diverse range of additional surfacecoupled reactive groups to be employed in molecular construction ofsurfaces for diverse applications.

Although the above descriptions have focused on the use of solutionreactions in the reactive coupling of targeted materials to the plasmamodified surfaces, as those schooled in the art will readily recognizethis second step coupling reaction could equally well be carried out byvapor phase reactions. For example, the reaction shown above (reaction4) could equally well be carried out using allyl alcohol vapor.Alternately, the coupling process could initially utilize an initialplasma deposition of allyl alcohol to provide surface hydroxyls whichare thus reacted with acryloyl chloride vapor, as shown below:

In this latter reaction, use is made of the high volatility of acryloylchloride relative to allyl alcohol. Obviously, the vapor phase couplingreactions could include a rich and diverse range of reactants includingvariations in the surface attached groups and the reactive functionalgroups of the vapor phase materials. As in the solution couplingreactions, the temperatures of the system would be adjusted to providereaction rates of desirable proportions.

Based on the above description, it is clear that this procedure can beemployed to introduce a multitude of surface functionalities havingvarying degrees of reactivity. By carrying out sequential derivatizationreactions, a molecularly tailored surface having a number of differentcovalently bonded molecules is readily created. Allyl derivatives arepreferred for the plasma depositions of the present invention because oftheir ready availability, volatility and low costs. They are not howeverthe only possible materials. Any volatile carbonaceous compound havingan active functional group may be used.

A major advantage of the use of the plasma process to introduce thereactive surface groups during the initial surface treatment is thatthis approach permits surface tailoring of virtually any solidsubstrate. Additionally, the plasma process provides uniform, pin-holefree coatings and they can be applied to any solid without regard togeometric considerations. The present inventors have successfullyapplied this coating procedure to solids such as polymers, ceramics(including glass), silanized glass, fabrics, paper, metals, silanizedmetals, semiconductors (e.g., silicon) carbon and even hydrogels.

A problem frequently encountered with plasma deposited coatings is pooradhesion of these films under low energy plasma conditions such as thoseencountered during low duty cycle pulsed and low energy plasmaconditions. While such films may have uses of their own, separate from asubstrate, and to that extent are a part of the present invention,adhesion to the substrate is a preferred aspect of the presentinvention. The present invention involves the discovery that theadhesion of these plasma films to the underlying substrates can bedramatically improved via use of a gradient layered technique. In thisprocess, the plasma deposition is initiated at a high RF duty cycle anda high RF power to provide an underlying initial coating stronglygrafted to the solid substrate. The RF duty cycle of the pulsed plasmais then progressively decreased, with this decrease providing increasingretention of the monomer functional groups, as shown in FIG. 1A-1D foran allyl bromide film. In this way, the successive plasma depositedfilms are tightly bonded to each other, providing a layered structure inwhich a gradient of monomer functional groups is present. The process isterminated at the lowest RF duty cycle needed to introduce a requiredsurface density of the reactive functional groups. Alternately, thegradient layering technique can be carried out under CW conditions withthe RF power being progressively decreased during the plasma depositionprocess. In this way, a strongly adherent film is deposited with theoutermost layer containing a high surface concentration of the desiredreactive functional group. This gradient layering technique has beensuccessfully employed to deposit plasma films on a wide range of solidsubstrates. The term “film” as used herein, those of skill in the artrecognize that this may but does not necessarily mean an intact film inthe usual sense and may vary in thickness, as a film, from 1 to 3000Angstroms or may be more widely dispersed non-interacting pendantgroups. These films are sufficiently well anchored to the underlyingsubstrates to preclude delamination when subjected to prolongedimmersion in various solvents, particularly including pure water andaqueous solutions.

Although the gradient layering technique described above has been foundto be useful in improving adhesion of plasma deposited films to varioussubstrates, there are other substrates (e.g. glass, metal and silicon)inwhich delamination of the gradient layered plasma films is observedafter prolonged immersion of the coated substrates in solution. Animportant aspect of the present invention is the discovery that thisdelamination problem can be solved via use of a plasma depositedsub-layer which helps to bridge the inherent incompatibility encounteredbetween an inorganic substrate (such as glass, metal or silicon) and thegradient layered organic plasma deposited coating. An example of thissub-layer technique is silanizing, the use of a plasma deposited filmfrom hexamethyldisiloxane (HMDSO) to successfully anchor aplasma-generated organic film (e.g. a film from allyl amine or allylbromide) to an inorganic substrate (e.g. silicon or glass). In theseexperiments, the organic plasma film was deposited using the gradientlayering technique described earlier.

The insertion of the HMDSO intermediate sub-layer between the glass,silicon or metal surface and the carbonaceous outer layer functionalizedplasma films works remarkably well in strongly anchoring these outerorganic films to the non-organic substrates. For example, films whichpreviously delaminated from these non-organic substrates a few minutesafter sample immersion in aqueous solution, have been observed to remainstrongly bound to the substrates, without noticeable changes in chemicalcomposition, after many weeks of immersion when the HMDSO sub-layer ispresent.

As those schooled in the art will recognize, the effectiveness of theintermediate HMDSO sub-layer in improving the adhesion of the organicfilms to the chemically different solid substrates is an example of butone of many sub-layers which might be employed to achieve this goal. Inparticular other organo-silicon compounds, such as silanes and othersiloxanes are expected to be particularly effective in this application(either being used to “silanize” a solid surface of the presentinvention). Other monomers which are capable of bridging the chemicaldissimilarities between organic and inorganic materials should alsoprovide this important function. Examples of such monomers would includevolatile organo-metallic compounds such as tetramethyl tin, furocene,tetramethyl lead, etc. which are known to provide organo-metallic filmsunder plasma polymerization conditions.

A further aspect of the present invention is to employ the low dutypulsed or low power CW plasma process to deposit fluorocarbon films ofexceptionally high hydrophobicity on solid substrates. These ultra lowenergy surface coatings can be employed directly for specificapplications or they could be coupled with simultaneous deposition of areactive functional group by combining the fluorocarbon monomer with anappropriate functionalized monomer. In the latter case the plasmagenerated surface consists of highly hydrophobic regions, except forthose locations occupied by the reactive functional groups. Thesereactive groups are available for subsequent chemical derivatizationreactions as described previously.

An aspect of the present invention is the discovery that the use of ahighly —CF₃ substituted fluorocarbon monomer can yield exceptionallyhydrophobic surfaces via plasma deposition. For example, utilizing lowduty cycle RF plasma deposition it is possible to retain, to a very highdegree, the —CF₃ content of the starting monomer. An example of this—CF₃ retention is shown in FIG. 2 which provides high resolution C(1s)ESCA spectra of fluorocarbon films obtained from an isomeric mixture ofsubstituted perfluorohexenes. As shown in FIG. 2, there is a clear-cutincreased retention of the —CF₃ groups of the starting monomer as the RFduty cycle employed during the deposition was decreased. Alternately,the same general increased —CF₃ retention can be achieved under CWplasma deposition conditions by reducing the RF power. These effects areclearly shown in FIG. 3.

FIG. 2B shows the ratio of the relative peak areas of CF₃ to CF₂ asbeing approximately 1.19, and FIG. 2C shows the ratio of relative peakareas of CF₃/CF₂ as being about 3.15; while FIG. 3B shows a ratio ofrelative peak areas of CF₃/CF₂ of about 2.17. Thus, the CF₃/CF₂ carbonratio ranges from about 1.2 to about 3.2.

Using the plasma deposition approach and the highly —CF₃ substitutedmonomer it is possible to generate a film whose hydrophobicity exceedsthat of conventional Teflon® surfaces. (see FIG. 4.) Both the advancingand receding water contact angles are higher than those observed with aconventional Teflon® (i.e., —CF₂ dominated) fluorocarbon film. (see FIG.5).

Although the above discussion has been illustrated with perfluorohexenemonomers, those skilled in the art will readily recognize that otherhighly —CF₃ substituted monomers (such as, e.g., other volatileperfluorinated compounds, particularly perfluorocarbons) can be employedunder low energy plasma conditions to provide —CF₃ dominated films.Either the low duty cycle pulsed plasma or the low power CW plasma wouldbe useful in providing retention of the original —CF₃ monomer content inthe plasma generated films.

By incorporating an appropriate functionalized monomer in with a highly—CF₃ substituted monomer, the plasma deposition process may be used toproduce a surface coating which is hydrophobic but at the same time,susceptible to covalent attachment of other molecules at the reactivefunctional group site. The extent of surface coverage by this functionalgroup is easily adjusted by variation of the relative concentration ofthe added functionalized monomer in the gas mixture employed.

In one particular embodiment of this invention a functionalizedfluorocarbon surface capable of ready attachment of other molecules isprepared using a functionalized fluorinated monomer which is added tothe heavily —CF₃ substituted primary monomer. In this way the entiresurface coating consists of carbon and fluorine atoms with the exceptionof the sites occupied by the reactive functional groups. An illustrationof this invention is plasma deposition of a mixture ofbromotrifluoroethylene and the substituted perfluorohexene mixture notedpreviously. The resulting film is extremely hydrophobic, highlynonadsorbent for biological molecules and chemically inert except forthe sites occupied by C—Br bonds. The C—Br can then be used to bindother molecules to the surface, using a simple one-step couplingreaction.

Surfaces modifiable by these processes include (but are not limited to)polymers, glass, ceramics, carbon, fabrics, paper, metal,semiconductors, wood, composites, cellulose, films (particularlypolymeric films) and hydrogels.

EXAMPLE 1

A variable duty cycle pulsed RF plasma was employed using allyl bromidemonomer to deposit a thin film containing C—Br bonds on the surface of apolymer (Dacron®) substrate. This film was deposited using an RF peakpower of 200 watts and an initial RF plasma duty cycle of 3 ms on and 5ms off. The monomer pressure was˜35 m Torr and the flow velocity was 3.5cm³/cmin (STP). After 3 minutes of plasma operation, the RF duty cyclewas reduced to {fraction (3/15)}, then {fraction (3/45)} and finally{fraction (3/60)} (plasma on to plasma off times, in ms). This procedureprovided a gradient layered film with good adhesion to the underlyingDacron® [polyethyleneterephthalate, (PET)] substrate and with arelatively high density of surface C—Br groups. (Surface Br atom contentwas 26% relative to 74% carbon atoms). Subsequently this surfacemodified sample was immersed in an aqueous solution which contained theamino acid hexafluoro-DL-valine at a concentration of 20 mM. After 10hours reaction at room temperature and a pH of 8.0, the coated Dacron®sample was removed, rinsed many times (including washing with asurfactant solution containing dissolved 1% sodium dodecyl sulfate(SDS), vacuum dried and then subjected to ESCA analysis. The ESCAanalysis revealed clearly the presence of the trifluoromethyl groups ofthe fluorovaline along with the presence of surface nitrogen from theamine group and oxygen from the carboxylic acid group. The highresolution C(1s) ESCA spectrum of this sample is shown in FIG. 6. Theseresults reveal that the molecular tailoring of the surface, in this casethe addition of the amino acid hexafluorovaline, was successfullyachieved via the simple two-step process involving plasma depositionfollowed by the room temperature derivatization reaction.

EXAMPLE 1A

A pulsed plasma process was employed to deposit a bromine-containingorganic film on a PET substrate. However, in this work dibromomethane,CH₂Br₂, was employed as monomer gas in lieu of the allyl bromide monomerused in Example 1. Again a gradient layering technique was employed toprovide good adhesion between the plasma deposited films and theunderlying solid substrate. The overmost layer of the plasma generatedfilm contained 19 atom % Br relative to carbon atom content, as shown byESCA analysis. This sample was then immersed in a 20 mM cysteine aqueoussolution at pH 8.0 and room temperature for 24 hours. Subsequently thesample was removed from solution, subjected to thorough rinsing with SDSsolution and distilled water, and then vacuum dried. The sample was thensubjected to ESCA analysis which revealed the presence of S, N and Oatoms on the surface indicating the presence of covalently boundcysteine molecules. However the relatively concentration of surfacebound cysteine molecules was notably less than that achieved with allylbromide films. This example clearly indicates that the monomer precursoremployed for the plasma surface modification does not have to be of thealkenic unsaturated type in order to provide reactive surface functionalgroups which can bind covalently to solute molecules. In this case, themolecular tailoring procedure was achieved employing an alkyl bromideduring the plasma modification step.

EXAMPLE 2

The same plasma treatment described in Example 1 was applied to apolished silicon substrate, in lieu of the Dacron® sample employed inExample 1. However, before deposition of the gradient layered brominecontaining film, a thin sublayer film of hexamethyldisiloxane (HMDSO)was plasma deposited on the Si sample (silanization). This sample wasthen derivatized as in Example 1, using the hexafluoro-DL-valinereagent. The same results as shown in FIG. 6, within experimental error,were obtained, indicating that the molecular tailoring achieved by thisinvention is independent of the nature of the solid substrate.Furthermore, the entire assembly was stable towards immersion in aqueoussolution with no evidence of film delamination after prolonged testing.

EXAMPLE 3

A gradient layered plasma generated film was again deposited on aDacron® (PET) substrate using allyl bromide monomer, as described inExample 1. This C—Br containing surface was subsequently immersed in anaqueous solution containing cysteine. This amino acid was chosen as itcontains sulfur as a distinctive label. ESCA analysis of this film afterderivatization and thorough rinsing reveals clearly the presence ofsurface attached sulfur, nitrogen, and oxygen atoms. This result clearlyreveals attachment of the cysteine molecules to the surface during thederivatization reaction.

EXAMPLE 3A

A control experiment was carried out in which an untreated PET substratewas immersed in an aqueous solution containing 20 mM cysteine at pH 8.0and room temperature. After 24 hours immersion this sample was removed,rinsed with SDS solution and distilled water and then vacuum dried.Subsequent ESCA analysis of this film revealed negligible surfacecontent of sulfur or nitrogen atoms. This result shows that successfulcoupling of the solute molecules to the substrate surfaces requires thepresence of reactive functional groups as introduced by the plasmadeposition treatment. It also affirms that the solute molecule surfacecoupling described in the Examples of this invention cannot be simplephysical adsorption phenomena as no solute molecule presence isdetectable on the unmodified PET substrate.

EXAMPLE 4

A sample prepared as in Example 1 was subjected to reaction with asolution containing the peptide YIGSR in place of hexafluoro-DL-valine.Subsequent ESCA analysis of this derivatized surface revealed clearlythe presence of amine and carboxylic acid groups consistent with theattachment of the YIGSR peptide. The ESCA spectra obtained afterattachment of YIGSR molecules is shown in FIG. 7A and FIG. 7B which showthe presence of —COOH groups at the high binding energy peak in theC(1s) spectrum [i.e., at 289eV] and the N(1s) ESCA peak (inset).

EXAMPLE 4A

In a separate experiment, the peptide RGD was also shown to bindcovalently to a PET substrate initially modified with a plasma depositedallyl bromide film as described in Example 1. Again ESCA analysis of thesurface after 10 hour aqueous solution reaction of RGD with the C—Brsurface groups was employed to rove the surface attachment of RGDmolecules as shown by surface N and O atoms.

The results of Examples 4 and 4A clearly illustrate the utility of thissurface tailoring procedure to bind any peptide sequence to the surfaceof the solid substrates.

EXAMPLE 5

A series of samples were prepared in which acryloyl chloride(CH₂═CH—COCl) was plasma deposited onto Si substrates. These depositionswere carried out over a wide range of plasma on and plasma off times, aswell as under CW conditions. Experimentally it was observed that thecomposition of the plasma films obtained varied dramatically with the RFduty cycle and/or the RF power density employed during the deposition.In particular, there was a clear-cut and very large scale increase inthe degree of retention of the acid chloride group (—COCl) as the RFduty cycle and power were decreased during the deposition. Thisobservation is illustrated in FIGS. 8 and 9. As shown by the highresolution C(1s) spectra in FIG. 8 there is a dramatic increase in therelative surface abundance of the —COCl group (i.e. the peak 289.7 eV)as the RF duty cycle was decreased in this series of runs at 25 wattpeak RF power. Also, as shown in this Figure, essentially no —COClretention was observed in the film deposited at 25 watt CW. FIG. 9provides vivid evidence for the increased surface incorporation of —COClgroups as the average power during plasma deposition is decreased.

In particular, there is a striking increase in retention of thesesurface COCl groups as the average power employed drops below 5 wattsand, particularly, below 1 watt. For the sake of completeness, FIG. 10provides a complete functional group assignment of the deconvolutedC(1s) high resolution ESCA spectra obtained from the acryloyl chloridemonomer. As the results in FIGS. 8 and 9 clearly evidence, it ispossible to both introduce and control the surface density of thesereactive —COCl groups using this pulsed plasma and/or low energy CWsurface modification technique.

The acid chloride group accounted for some 11% of the total carboncontent of the surface layer. Subsequently this film was immersed in apH 7.4 buffered solution of water. After four hours immersion, ESCAanalysis revealed slightly increased oxygen atom incorporation in thefilm, showing that a small degree of hydrolysis of the surface coatinghad occurred. This sample was then immersed in an aqueous solution whichcontained cysteine. ESCA analysis of this film after four hour reactionin this amino acid solution and thorough rinsing clearly revealed thepresence of surface attached cysteine molecules as shown by the sulfurand nitrogen ESCA signals. The cysteine surface attachment was slightlyhigher than that obtained previously with the bromine containingsurface.

EXAMPLE 6

A sample was prepared in which a pulsed plasma, 10 μs on and 1000 μsoff, was employed to deposit an outer film rich in —COCl groups on a PETsubstrate using the acryloyl chloride monomer. The deposition processinvolved a gradient layering technique in which an initial high RF dutycycle was gradually decreased to the aforementioned 10/1000 (μs) value.ESCA analysis of the surface of this sample revealed high —COClconcentration in which this acid chloride group accounted for some 11%of the total surface carbons.

EXAMPLE 7

Pulsed plasma deposition studies of acrylic acid (CH₂═CHCOOH) werecarried to provide surface active —COOH functional groups. As noted inprevious examples, reduction of the RF duty cycle during depositionleads to increasing incorporation of —COOH groups in the plasmadeposition films. This is shown in FIG. 11 in which the high resolutionC(1s) ESCA spectra from the acrylic acid plasma film are shown atdifferent RF duty cycles. Clearly, there is a rapid increase in the—COOH content of these films (i.e., the peak at 290 eV) as the RF dutycycle employed during deposition was decreased. Additionally, theincreased —COOH content of these films is observed by FT-IR absorptionspectra of the plasma generated films as shown in FIG. 12.

Plasma synthesized films of acrylic acid (containing surface active—COOH groups) were subsequently subjected to chemical derivatizationreactions. Successful covalent attachment of amines (via formation ofamide groups (and alcohols (via formation of ester groups) weredemonstrated by surface analysis of these films after the derivatizationreactions. These reactions were carried out in both aqueous andnon-aqueous solvents.

EXAMPLE 8

A pulsed plasma deposition of allyl bromide was employed to deposit anadherent thin film containing C—Br bonds on a glass substrate. The HMDSOsub-layer and gradient layer of bromide film, as described earlier, wereemployed to improve the adhesion of this film to the glass substrate.This sample was then immersed in a 20 mM aqueous solution of the aminoacid proline for 8 hours at room temperature and a pH of 8.0.Subsequently this sample was rinsed thoroughly with distilled water, 1%SDS solution, more distilled water and then vacuum dried. ESCA and FT-IRanalysis of this film revealed the presence of both nitrogen (i.e. aminegroups) and carboxylic acid groups now present on the surface of thissample, consistent with the attachment of the proline molecules. Thisprovides a further example of the general utility of this surfacemodification procedure in that a cyclic molecule (i.e., proline) wassuccessfully attached to the surface. This example also illustrates theutility of the HMDSO plasma deposited sub-layer film to improve theadhesion of subsequent deposited films to the glass substrate. Inseparate tests, no delamination of the plasma deposited C—Br films werenoted after prolonged (i.e. over 3 weeks) immersion of the glass coatedsamples in aqueous solution at room temperature.

EXAMPLE 8A

A stainless steel (315) substrate was initially coated with an HMDSOsub-layer plasma film followed by the deposition of a gradient layeredallyl amine film. This sample was then immersed in aqueous solution. Nodelamination or film compositional changes were observed after prolongedimmersion, as shown by ESCA analysis of the —NH₂ surface film before andafter aqueous solution immersion.

The combination of examples 8 and 8A are provided o document theefficacy of a HMDSO sub-layer in improving the adhesion of plasmadeposited carbonaceous films, including those deposited at low plasmapowers, to solid inorganic substrates.

EXAMPLE 9

A pulsed plasma deposition of a trimer mixture of perfluorinatedsubstituted hexenes (C₉F₁₈) was employed to a deposit a fluorocarbonfilm on a silicon substrate. The CF₃ content of the plasma depositedfilm can be controlled via the RF duty cycle employed during thedeposition process. This film chemistry controllability is clearlyillustrated in FIGS. 2A-2C, which shows increasing CF₃ filmincorporation as the RF duty cycle employed during the deposition isdecreased. For example, a film deposited at a RF duty cycle of 0.1 msplasma on and 3.0 ms plasma off and 100 watts peak power consists of acomposition in which 40% of the surface carbon atoms are present as CF₃groups. This film is exceedingly hydrophobic with a surface energy whichis even less than of —CF₂— dominated surfaces (such as Teflon®) as shownby water contact angle measurements (see FIGS. 4 and 5). The CF₃—dominated film structure, in fact, represents a unique new form offluorocarbon polymer. The advancing water contact angle was measured tobe 127°.

The general reaction system employed in the present work has beendescribed previously (Panchalingam et al., 1993; 1994). The C₉F₁₈ wasobtained from PCR™, Inc. (Gainesville, Fla.) and consisted of a mixtureof three perfluoro-compounds: 2,3,5-trimethylhexene-3;2,3,5-trimethylhexene-2; and, 2,4,5-trimethylhexene-2. This mixture wassubjected to thorough degassing via freeze-thaw cycles but was notsubjected to any further purification. All plasma runs were carried outat a C₉F₁₈ pressure of 50±2 m Torr and a flow rate of 5.05±0.05 cm³/min(at STP). The plasma generated thin films were deposited on polishedsilicon substrates and they were subsequently characterized by XPSanalysis. These films were also evaluated using the sessile drop watercontact angle approach (Rame-Hart type goniometer).

Samples were prepared using both pulsed plasma and continuous-wave (CW)plasma operation. The pulsed plasma depositions involved RF duty cycles(i.e., ratio of plasma on to plasma off times, in ms) of 10/30 and10/300 at 200 watts peak power and 0.1/3 at 100 watts. The CW runs werecarried out at 50 and 5 watts. In terms of equivalent wattages, thepulsed runs approximate the total power of the CW runs when averagedwith respect total elapsed times. For example, the pulsed deposition at10/30 and 200 watts corresponds to an equivalent (or average) power of50 watts (i.e., 10/40×200).

The XPS film analyses are summarized in Table I and FIGS. 2A-2C (pulsedruns) and FIGS. 3A-3B (CW runs). The C(1s) XPS peak assignment shown arebased on the accepted peak identities for fluorocarbon films (Clark andShuttlerworth, 1980). As shown in Table I, the F/C atom ratio in thefilms increases with a decrease in average power under both pulsed andCW conditions. The atom ratios shown in Table I were computed from thedeconvoluted high resolution C(1s) XPS peaks as opposed to the directlymeasured F to C values provided by integration of the respective F(1s)and C (1s) peaks. These latter values show the same trends in F to Cratios as reported in Table I. However, they provide values as high as2.26, which appear to be unreasonably high considering the structures ofthe C(1 s) peaks. Others have observed comparable results in contrastingthe F to C ratios from deconvolution of C(1s) XPS peaks with the valuesobtained from the separate F(1s) and C(1s) signals (Clark andShuttlerworth, 1980). As shown in Table I and, as is clearly evident inFIGS. 2A-2C and FIGS. 3A-3B, there is a notable increase in —CF₃incorporation in the film synthesized at a 0.1/3 duty cycle and 100watts. The CF₃ groups represent 40% of the total carbon content of thefilm. This compares to a theoretical maximum of 55% present in thestarting monomer mixture.

TABLE I RELATIVE F, C, AND —CF₃ CONTENT OF PLASMA POLYMERIZED C₉F₁₈MONOMERS. THE RELATIVE F AND C ATOM CONCENTRATIONS WERE COMPUTED FROMDECONVOLUTION OF THE C (1s) XPS PEAK. on time/off time power (ms)(watts) % F % C % O F/C % CF3 Pulsed runs 10/30 200 53.59 45.34 1.071.18 15 10/300 200 57.92 41.53 0.55 1.39 24 0.1/3 100 62.90 36.60 0.501.72 40 CW Runs 50 56.78 42.81 0.41 1.33 19 5 60.78 38.40 0.82 1.58 34

The hydrophobicities of these unusual fluorocarbon films were evaluatedvia both advancing (θ_(a)) and receding (θ_(r)) water contact anglemeasurements. Examples of the results obtained are shown in FIG. 4, inwhich θ values from a —CF₃ dominated films are contrasted with valuesfrom a standard Teflon® film (Goodfellow Inc.) sample. The θ values havean estimated uncertainty of ±3 degrees. As shown in FIG. 4, both theθ_(a) and θ_(r) values for the 0.1/3 duty cycle pulsed plasma generatedfilms are significantly higher than those obtained for the Teflon® filmsample. The 5 watt CW synthesized film exhibited θ_(a) values which wereslightly less than those of the Teflon® film. Additionally, the θ_(r)values for this CW generated film exhibit a slightly higher degree ofhysteresis than the other two fluorocarbon samples. The higher θ valuesfor the pulsed plasma generated films would be in accord with the higher—CF₃ content and the higher F/C ratios in this film relative to the 5watt CW sample (Table I). The inventors also note that the filmsdeposited during the 0.1/3, 100 watt pulsed depositions differ slightlyfrom the lowest power CW runs with respect to the relativeconcentrations of carbon atoms not bonded directly to fluorine (peaks at286.5 eV, FIG. 2 and FIG. 3). These particular carbon atoms appear to beslightly more prominent in the CW generated films and this factor mayalso contribute to the differences in hydrophobicities of these twosamples. The differences in these peak intensities may arise fromenhanced fluorine atom ablation processes under CW conditions relativeto those observed under pulsed operation.

The XPS spectra in FIG. 2 and FIG. 3 confirm that increased destructionof the —CF₃ groups is observed under more energetic plasma conditionsunder both pulsed and CW conditions. This leads to more highlycross-linked film as shown in these figures. This observation is inaccord with literature results of previous studies of fluorocarbon films(Yasuda, 1985).

The lowest duty cycle 100 watt pulsed plasma generated film haveslightly higher —CF₃ content and are more hydrophobic than the lowestwattage CW prepared films. Attempts to generate films at even lessenergetic conditions, under both pulsed and CW conditions, resulted inthe relatively unstable plasmas and/or exceptionally low depositionrates. Conceivably, lower energetic conditions, when coupled with othermonomer processes and flow rates, might provide slight enhancements in—CF₃ film content. However, the high retention of the monomers' —CF₃groups obtained in the present study indicates there is probably littleroom for generation of even more hydrophobic films using this particularmonomer mixture.

The availability of these extremely hydrophobic fluorocarbon films, withtheir high —CF₃ content, provides a new and more richly fluorinatedsurface. Significant decreases in plasma protein adsorptions wereobserved on these —CF₃ dominated surfaces, relative to those previouslyreported for —CF₂-structured surfaces. Other important distinctionsbetween the properties of —CF₃, relative to conventional —CF₂—, surfaceswill emerge as comparative studies progress.

Previously, the inventors have reported on film chemistry control duringpulsed plasma synthesis of fluorocarbon films (Savage et al., 1991;Panchalingam et al., 1993). In these studies, relatively large scaleprogressive changes in film compositions were observed with sequentialchanges in the RF duty cycle employed during plasma polymerization ofseveral perfluorocarbon monomers, all other plasma variables being heldconstant. These fluorocarbon films exhibited a steady increase in—CF₂-functional content as the plasma duty cycle was reduced, resultingin unusually low surface energy films at the lowest duty cyclesemployed. This trend towards higher —CF₂-film incorporation wasdemonstrated both with monomers containing high initial —CF₂-content(e.g., perfluoro-2-butyl tetrahydrofuran) as well as with monomershaving less abundant —CF₂-groups (e.g., perfluoropropylene andhexafluoropropylene oxide).

The present invention comprises the first evidence for the controlledplasma synthesis of perfluorocarbon films dominated by —CF₃ groups.These exceptionally hydrophobic films were produced by plasmapolymerization of a mixture of —CF₃ substituted perfluorohexenes [trimerof C₉F₁₈ compounds]. This starting isomeric mixture is dominated by —CF₃groups functionalities which account for 55% of the carbons present inthese molecules. Additionally, these molecules contain a carbon-carbondouble bond useful in helping to promote polymerization, particularlyunder low energy plasma conditions.

EXAMPLE 10

The same perfluorocarbon substituted trimer mixture noted in Example 9was mixed with a bromine-containing monomer. Plasma deposition produceda fluorinated film that also contained C—Br reactive surface groups asshown by ESCA analysis.

EXAMPLE 11

A number of PET substrate surfaces were modified using a pulsed plasmatreatment process and allyl bromide monomer, as described previously inExample 1. These samples were employed for a time study of the couplingof cysteine molecules to the surfaces of this C—Br containingsubstrates. The extent of surface attachment of the cysteine moleculesas a function of reaction time was determined by ESCA analysis of thesurface N and S content. Reactions were carried out in aqueous solutionat pH 7.4 and room temperature. It was observed that the majority of thecysteine attachment to the surface occurred during the first 10 hours ofreaction. This result is shown in FIG. 13 which shows the relativesurface content of nitrogen atoms as a function of the time of reaction.This result clearly shows the efficiency of the amino acid couplingreaction to the plasma modified surface even where this reaction iscarried out at room temperature.

EXAMPLE 12

A pulsed plasma deposition of allyl iodide was employed to provide asurface which contained carbon-iodine (C—I) bonds. As in previousexamples, it was observed that the relative surface content of the C—Ibonds increased with decreasing energy employed during the depositionstep. Subsequently, these C—I containing films were subjected toderivatization reactions utilizing glutamine and cysteine as couplingmolecules. It was observed that the amino acids attach to these surfacesas revealed by the presence of N and S via ESCA analysis. However, theamounts of N and S incorporated were less than that observed with theC—Br and C—COCl modified surfaces (Examples 3 and 4, respectively). In aseparate experiment a PET modified surface containing plasma depositedC—I groups was immersed in pure H₂O for six hours and that subjected toESCA analysis. This analysis revealed a decrease in surface I contentand increased O content relative to the concentrations before H₂Oimmersion. This experiment indicates that C—I hydrolysis is occurringduring the room temperature immersion experiment and that thishydrolysis process is competitive with the reaction of the C—I groupswith the cysteine molecules. Use of more concentrated amino acidsolutions and/or lower reaction temperatures would be expected to helppromote additional amino acid coupling to the surface and reducedhydrolysis.

EXAMPLE 13

A pulsed plasma surface modification was employed to introduce surfaceC—Br groups, as described previously in Example 1. This sample was thenimmersed in an aqueous solution which contained dissolved NH₂-PEG-OCH₃polymer molecules. The functionalized PEG polymeric molecules had anominal molecular weight of 5000 and were purchased from ShearwaterPolymers, Inc. After 24 hours of reaction, these PET samples wereremoved, rinsed with SDS solution and distilled water and then vacuumdried. Subsequent ESCA analysis of these films revealed clearly thesurface attachment of the functionalized PEG polymer molecules. TheC(1s) high resolution ESCA spectrum obtained after this couplingreaction is shown in FIG. 14. Table II provides a quantitative measureof the surface atom content before and after the derivatizationreaction.

TABLE II ESCA C(ls) Analysis of Surface Atoms on C—Br Modified SurfacesBefore and After Attachment of NH₂—PEG—OCH₃ C Br O Before Derivatization0.74 0.26 After Derivatization 0.71 0.11 0.18

As shown in both FIG. 14 and Table II there is a substantial oxygen atompresence on the surface after derivatization. The position and bindingenergy of the C—O ESCA peak identifies this oxygen content as arisingform ether groups of the type involved in the PEG linkages. We also notethe decrease in surface Br atoms which accompanies attachment of theNH₂-PEG-OCH₃ molecules, as expected. We also mention explicitly that theN content of the derivatized surfaces is below the ESCA detectabilitylimit as calculated based on the PEG incorporation

Subsequently these PET-PEG modified surfaces were employed in proteinbinding studies using radioactively labeled (¹²⁵I) albumin molecules. Adramatic decrease in albumin surface adsorption (>30 fold decrease) wasobserved in comparing albumin adsorption on the unmodified PET substratewith that obtained on the PEG modified PET surface. The results obtainedclearly support the utility of the surface modification procedure of thepresent invention to provide a new and convenient approach tomanufacturing of non-biologically fouling surfaces.

EXAMPLE 14

A pulsed plasma deposition was employed as generally described inExample 9 to generate a PET modified surface having an extraordinaryhigh surface density of CF₃— groups, such as that shown in FIG. 2Ccontaining 40% CF₃ groups. This sample was then used to measure proteinadsorption (as in Example 13) vis-à-vis that observed on conventionalfluorocarbon surfaces. In general, a sharp decrease in proteinadsorption was observed on the CF₃— dominated films relative to thatobserved on other fluorocarbon samples. In particular, a two-foldreduction in albumin adsorption was noted on the CF₃— dominated surfacesrelative to those observed on CF₂— dominated (ie., Teflon) surfaces.This example illustrates the utility of the unique CF₃— surfaces inreducing non-specific biomolecule adsorptions. Thus, these CF₃— surfaceswhen coupled with an added functional group (such as C—Br as notedearlier) can be employed to reduce non-specific biomolecular adsorptions(e.g., blood platelet adhesion, protein binding, etc.) whilesimultaneously permitting attachment of specific molecules via thereactive functional groups.

Citations in the following list are incorporated in pertinent part byreference herein.

References

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Miyamoto et al., U.S. Pat. No. 5,178,962, 1993.

Panchalingam et al., 1993, J. Biomater. Sci., 5, 131.

Panchalingam et al., 1994, J. Appl. Polymer. Sci.; Appl. PolymerSymposium, 54, 123.

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Yasuda, 1986, Plasma Polymerization, P. 182, Academic Press, NY.

Those skilled in the art will perceive many equivalents of theprocedures, active group-containing monomers, perfluorinated compoundsand surfaces specified in the following claims.

What is claimed is:
 1. A solid surface having pendant perfluorinatedgroups where the CF₃/CF₂ carbon ratio range from about 1.2 to about 3.2.2. The solid surface of claim 1 where the CF₃ is at least 15 to 40% ofthe total carbon present.
 3. A surface having a pendant perfluorinatedcarbonaceous compound and a water contact angle greater than about 120degrees, wherein said pendant perfluorinated carbonaceous compound has aCF₃/CF₂ carbon ratio ranging from about 1.2 to about 3.2.
 4. The surfaceof claim 3 where the perfluorinated carbonaceous compound is CF₃substituted perfluorinated hexene.